Epizootic tularemia occurred in 723 rhesus macaques group-housed in six 1-acre breeding corrals in late winter and spring, 1996. Twenty-four deaths were attributed to the disease and included 6 adults greater than 3.5 yrs. of age and 18 juveniles and infants, including 3 stillborn infants. The seroconversion rate in the population was determined using a whole bacteria microagglutination test (MAT). Of 723 macaques, 185 (25.6%) were seropositive, MAT titer > 1:128. Of the 185 seropositive macaques, 49 (26.5%) seroconverted between May, 1990 and August, 1995 and 136 (73.5%) seroconverted after August, 1995. None of the sera available for 12 of the dead animals yielded positive MAT titers. Thus, new infections in live-born animals totaled 157 or 23.3% of the susceptible population with a mortality rate of 13.4%. Conception and abortion/stillbirth rates were analyzed in a corral group of 66 adult females with a low seroconversion rate prior to August, 1995 (6.1%), and a high new infection rate (47.0%). The expected conception and abortion/stillbirth rates adjusted for maternal age were 79.7% and 9.8%, respectively. The observed conception and abortion/stillbirth rates were 59.1% and 28.2%. Three of the stillbirths were in females that died of tularemia, 3 were in utero deaths several days prior to expulsion and 4 were dystocia-related. Attempts to isolate F. tularensis were unsuccessful. Tularemia was not recognized prior to the 1996 epizootic despite serologic evidence of infections between 1990 and 1995 nor is the area considered a high endemic area for tularemia. The organism has been partially characterized and appears to be a moderately pathogenic group B strain with a mouse LD50 of 1 organism. A reservoir has not been identified.